Heart disease is a major cause of mortality. There is a need for methods and apparatus that will permit the early detection of heart problems and for methods and apparatus capable of yielding information useful for diagnosing heart conditions.
Prior methods for the non-invasive monitoring of cardiac function have included:                Mechanical methods, for example, pulse recording of the jugular carotid artery or apex cardiography.        Electrical techniques, for example, electrocardiograms (ECGs).        Imaging techniques, including echocardiology, radiography and magnetic resonance imaging (MRI).        
Existing mechanical methods can be inaccurate because of physical differences between subjects. For example, the intensity of heart sounds is not a good measure of heart function because physiological differences between subjects, such as differences in thickness of layers of fat in the subjects, affects the intensity of heart sounds.
Electrical techniques suffer from the disadvantage that it is difficult to correlate the measured electrical signals with the force of cardiac contraction. Imaging techniques are also subject to this problem. For example, an echocardiogram determines a ratio known as the “ejection fraction”. In a normally-functioning heart the ejection fraction may be related to the force of the heart's contraction. However, if the heart is not functioning normally then this relationship may fail to hold true.
None of the above-mentioned prior methods or techniques can accurately characterize the isovolumic phase of the heart cycle. Characteristics of the isovolumic phase can be important in identifying coronary artery disease and other heart-related conditions.
Pinchak, ESOPHAGEAL ACCELERATION AND THE CARDIOVASCULAR SYSTEM, Journal of Sound and Vibration, 1979, pp. 369-373 evaluates the use of miniature accelerometers within a stethoscope.
Koblanski, U.S. Pat. No. 5,865,759, the disclosure of which is incorporated herein by reference, discloses an apparatus and method for assessing cardiac function in human beings. The apparatus provides a sensing mechanism positioned on the thyroid cartilage in the neck against the trachea for sensing a response of the thyroid cartilage to heart function. A restraining system is provided to hold the sensing mechanism in position. It has been found that the apparatus, while useful, has several disadvantages including:                the apparatus is undesirably sensitive to the posture of the subject;        the apparatus can fail to detect accurately low-magnitude heart motions that occur immediately after a larger-amplitude heart motion;        the signal-to-noise ratio is undesirably low;        properly adjusting the restraining system is undesirably difficult; and,        the system can provide erroneous results if parts of the system contact obstacles such as clothing, pillows, beards, fatty neck tissue, or the chest.        
There remains a need for practical methods and apparatus for monitoring heart motions.